Passive Tuned Vibration Absorbers (TVAs), otherwise referred to as tuned absorbers, proof mass devices, or tuned inertial devices, include an inertial tuning mass suspended by a spring-like member. The mass of the tuning mass and stiffness of the spring-like member are "tuned " such that the TVA generally reduces vibration of a vibrating member at the attachment point and at its resonant natural frequency (fn). The disadvantage of passive TVAs is they are only effective at a particular disturbance frequency (fn) or within a very narrow frequency range (effective range) thereabouts. Furthermore, TVAs may be too massive for applications where weight considerations are paramount. Although, in some applications, TVAs provide excellent vibration reduction, in other applications where the disturbance frequency exposed to the vibrating member varies widely, a passive TVA may perform unacceptably. This is particularly true outside of the TVA's effective range. U.S. Pat. No. 3,490,556 to Bennett, Jr. et al. entitled "Aircraft Noise Reduction System With Tuned Vibration Absorbers " and U.S. Pat. No. 3,668,939 to Schrader entitled "Plane Omnidirectional Absorber " are illustrative of passive TVAs for canceling vibration and/or noise. Further, SAE Paper 830736 to E. H. Waterman, D. Kaptein, and S. L. Sarin describes "Fokker's Activities in Cabin Noise Control for Propeller Aircraft " and describes passive absorbers attached to the fuselage frame. A similar paper by T. Bevan and R. Hund entitled "Vibration Absorbers Quiet Propeller Noise" describes passive vibration absorbers for reducing low frequency interior noise in Beech Aircraft.
When a wider frequency range of vibration cancellation is required, various adaptive controllable or multi-frequency TVAs have been employed. For example, U.S. Pat. No. 5,361,878 to Borsati entitled "Dynamic Two Frequency Vibration Damper " teaches a TVA which can be passively tuned to cancel vibrations at two different frequencies. U.S. Pat. No. 3,085,657 to Preis entitled "Variable Frequency Vibration Absorber " teaches a TVA where the position of the mass 14 on spring 10 can be adjusted via arm 16 to change the tuned natural frequency (fn) of the TVA. U.S. Pat. No. 3,430,902 to Lohr entitled "Variable Stiffness Polymeric Damper " teaches a TVA whose resonant frequency (fn) may be varied by controlling the temperature of the polymeric spring element 16 which has temperature dependent stiffness characteristics. Again, even adaptive controllable or multi-frequency devices may prove unacceptable in terms of performance or weight for some applications.
Where better vibration isolation or lower weight is desired, Active Vibration Absorbers (AVAs), otherwise referred to as inertial shakers, or active proof mass devices may be employed. AVAs include an active element that actively drives an inertial tuning mass through a wide range of frequencies and in various phase relationships to the disturbance. U.S. Pat. Nos. 5,427,362 to Schilling et al., U.S. Pat. No. 4,694,650 to Vincent, and U.S. Pat. No. 3,917,246 to Gartner et al. teach various AVA-like devices. U.S. Pat. Nos. 5,332,061 to Majeed et al., 5,310,137 to Yoerkie, Jr. et al., 5,251,863 to Gossman, and 4,715,559 to Fuller describe various systems where the AVA-like devices are useful. In particular, it is known to use AVA-like devices to cancel engine vibration in automobiles, to attach directly to the inside surface of the fuselage of an aircraft to cancel noise therein, to cancel high-frequency vibrations of a helicopter transmission, and to cancel vibrations in machinery.
U.S. Pat. No. 5,439,082 to McKoewn et al., commonly assigned U.S. Pat. Nos. 5,427,347 to Swanson and 5,174,552 to Hodgson et al., and U.S. Pat. No. 5,067,684 to Garngost, describe active mountings/isolators for attachment between a vibrating member and a structure. These active mountings differ from AVAs in that they "attach between two elements" and provide active forces "between two elements" and, therefore, have two connection points. One connection is to structure and the other to the vibrating member. On the other hand, AVAs are inertial devices that only attach at a single point, i.e., at the point where the vibration is to be controlled and include an inertial tuning mass that is flexibly-suspended. AVAs include a tuned resonant frequency where the system is designed to be operated at, or near.
Commonly assigned U.S. Pat. No. 4,811,919 to Jones describes a volume compensator used in a fluid mounting for allowing expansion of fluid contained therein and for providing pressurization thereof.
U.S. Pat. No. 5,231,336 to van Namen, describes an actuator for active mountings which includes multiple coils which generate magnetic fields in opposition to one another when energized.
It should be understood that none of the afore-mentioned devices provide a compact and efficient AVA which has an internal means for driving a primary mass and which incorporate at least one fluid chamber including a volume stiffness and use fluid as an intermediate driving medium to drive said primary mass, and which include a primary resonance and preferably a secondary resonance which may be used to provide efficient control of vibration at multiple frequencies.